LearnI/O Redirection: >, >>, 2>, 2>&1, &>, /dev/null & Here-Strings

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I/O Redirection: >, >>, 2>, 2>&1, &>, /dev/null & Here-Strings

In Plain English

Every process in Linux is assigned three default I/O file descriptors: Standard Input (`0: stdin`), Standard Output (`1: stdout`), and Standard Error (`2: stderr`). Redirection operators control where program output goes (files, null devices, or other streams).

The Mental Model

"A command is a machine with 1 input funnel (0) and 2 output spouts: blue for normal products (1: stdout) and red for error alerts (2: stderr). Redirection hooks hoses onto those spouts to direct flows to specific buckets."

Deep Dive: How It Works

File Descriptors: `>` (redirect stdout / truncate), `>>` (redirect stdout / append), `2>` (redirect stderr), `2>&1` (merge stderr into stdout), `&>` (redirect both stdout and stderr).

The `/dev/null` Black Hole: Discard unwanted noisy output by redirecting to `/dev/null` (e.g. `cmd > /dev/null 2>&1`).

Here-Strings (`<<<`): Pass a string variable directly to standard input: `grep "pattern" <<< "$MY_VAR"`.

Syntax Blueprint

command > output.log 2> error.log   # Separate logs
command >> app.log 2>&1             # Combined append
command > /dev/null 2>&1            # Completely silent
base64 -d <<< "WmVubGFi"            # Here-string input

Use 2>&1 to merge error streams and /dev/null to silence command output.

Core Rules to Remember

Order Matters in 2>&1: `> file 2>&1` directs stdout to file, then points stderr to the same place. Writing `2>&1 > file` fails to merge.

Here-Strings <<<: Feeds strings into commands without spawning an extra `echo |` process.

Input Redirection <: `wc -l < file.txt` feeds file as pure stream without printing filename.

Common Beginner Traps & How to Fix Them

Mistake:Thinking `2>&1` and `&>` behave identically in legacy `/bin/sh` POSIX scripts.

Why it happens: `&>` is a Bash extension; standard POSIX `sh` requires `> file 2>&1`.

How to fix: Use `> file 2>&1` for universal cross-shell compatibility.

Pro Tip: Use `exec 3>&1 4>&2` to create custom custom file descriptors for advanced script logging.

Live Interactive Example

Hit Run Code to see it live

Separating and Merging stdout and stderr Streams

Git 2.45 / CLI

Output Console

Click "Run Code" to view the rendered output.

How it works: >&1 sent to stdout and >&2 routed the warning to stderr.log.

Your Turn: Micro Challenge

No pressure! Edit the starter code below and test your solution with instant feedback.

Micro Exercise

Feed Input with a Here-String

Use `tr 'a-z' 'A-Z' <<< "zencompiler"` to convert a here-string.

Sandbox Output

Click "Run & Check" to test your solution.

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